The purpose of this project is to improve understanding of the formation of harmful gas- phase pollutants, such as formaldehyde, and aerosol particles in the indoor work environment. Asthma, allergies and frequent work-related non-specific symptoms (i.e., irritation, headache and fatigue) are strongly associated with exposure to airborne pollutants in non-industrial workplaces. Chemical reactions involving ozone of outdoor origin and indoor materials are among the main sources of formaldehyde and other irritant gas-phase oxidation products and respirable aerosol particles in the indoor work environment. However, the chemical sources of many secondary indoor pollutants are poorly understood. There is a particularly critical need to characterize chemical mechanisms, rates and product yields of ozone reactions taking place on surface materials, in order to understand their link with health effects and implement mitigation strategies. This project is aimed at studying ozone reactions with particulate matter and sorbed chemicals collected on heating, ventilation and air conditioning (HVAC) filters. The study will also test the inertness of filter media. HVAC systems operate in office buildings, healthcare facilities, schools and other indoor workplaces in the US. Because they are so prevalent, HVAC systems could be a very significant and widespread source of indoor pollutants. HVAC filters are exposed to high ozone concentrations (close to outdoor levels) and are therefore very susceptible to ozone attack. The proposed research will utilize a flow tube apparatus to identify key constituents of particle-laden HVAC filters that react with ozone to generate volatile indoor pollutants and secondary organic aerosols. Used and unused sections of HVAC filters will be exposed to atmospheres containing various ozone and humidity levels in realistic operation conditions to explore the yield of formaldehyde and other airborne pollutants. Extraction, chemical analysis and infrared spectroscopy will be used to identify main chemical transformations taking place on the exposed surfaces. Subsequently, ozone reactions with key surface chemicals will be studied in individual lab-bench exposure tests to establish the reaction rates and mechanisms that lead to the formation of gas-phase and particulate pollutants, and to assess the effect of surface moisture content on pollutant yield. This information will be used to evaluate the overall impact on the indoor work environment of ozone chemistry with particle-laden HVAC filters. Asthma, allergies and frequent work-related non-specific symptoms are strongly associated with exposure to airborne indoor pollutants in the workplace. Harmful indoor air pollutants such as formaldehyde and ultrafine aerosol particles can be generated in ozone reactions with particle-laden filters in ventilation systems. In this application we propose to characterize the complex chemical processes taking place in ventilation filters exposed to ozone, and to evaluate the overall impact of these reactions on the indoor work environment.